1. Field of the Invention
The invention relates to semiconductor wafer processing systems that perform xe2x80x9cdryxe2x80x9d etching of semiconductor wafers. More particularly, the invention relates to a method and apparatus for performing in situ measurement of etch uniformity as a semiconductor wafer is etched by a semiconductor processing system.
2. Description of the Background Art
Semiconductor processing systems that perform xe2x80x9cdryxe2x80x9d etch of semiconductor wafers typically rely upon post processing analysis to determine the uniformity of the etch process. To use such post processing techniques, a plurality of wafers are etched and then the amount of material removed by the etch process is measured using depth-measuring equipment, such as a laser interferometer for transparent films or profilometer for opaque films. The measuring equipment measures the etch results at various points on the wafer surface. The relative depth of each point provides an indication of etch uniformity. Such wafer post processing is expensive and consumes many wafers before the uniformity produced by the etch process is satisfactory.
There has been some success in the art in developing in situ etch uniformity measuring systems that utilize optical emission spectroscopy to monitor light emissions from the plasma as the etch process progresses. One such system is disclosed in U.S. Pat. No. 5,362,356 issued Nov. 8, 1994 and incorporated herein by reference. This patent discloses a passive method of monitoring film removal (or deposition) during plasma etching (or deposition) based on interference phenomena. The system monitors plasma emission intensity at a selected wavelength, without additional illuminating apparatus, and variations in plasma emission intensity are correlated to remaining film thickness, etch rate, etch uniformity and etch selectivity. However, to derive a uniformity for a wafer being etched, the ""356 invention must make certain assumptions concerning the etch rate and the thickness of the wafer films in order to predict the uniformity at any point during the etch process. Such assumptions can be error prone.
Therefore, a need exists in the art for a method and apparatus for performing direct, in situ measurement of etch uniformity.
The disadvantages associated with the prior art are overcome by the present invention of a method and apparatus for performing direct, in situ measurement of etch uniformity. Specifically, the present invention analyzes optical emission spectroscopy (OES) data produced by an etch rate monitoring system. The invention computes the first derivative of the OES data as the data is acquired. When the OES data meets a predefined trigger criterion (i.e., a trigger point) such as attaining the etch endpoint or reaching an inflection point in the data, the invention correlates the value of the first derivative with a particular uniformity value. The correlation can be accomplished using a look-up table or a correlation equation. The look-up table or correlation equation are predefined using empirical data. The invention then displays the uniformity value and/or responds in some manner to the uniformity value. The present invention has been found to accurately predict etch uniformity using this direct, in situ measurement technique. As such, extensive post-processing is not necessary which substantially reduces the costs associated with performing an etch process.